Justification:
Listed as Critically Endangered because its extent of occurrence is less than 100 km², it is known from only one location, and there is continuing decline in the extent and quality of its habitat.

Until recently, this species was known only from the type locality; which is on the western slope of the extinct Volcán San Martín Tuxtla in Veracruz, Mexico (Hutterer 2005). It has been recorded from 1,463 m up to the summit of the volcano at 1,650 m asl (Goldman 1951). Recently, it was found at three additional localities in the vicinity of the type locality (Cervantes and Guevara 2009). These localities lie on the south face of the volcano and just to the north west of the nearby Catemaco Lake (Cervantes and Guevara 2009). It has been hypothesized that this shrew probably occurs in suitable habitats throughout that Sierra (Choate1970). However, a recent study on the mammalian diversity of Sierra de Santa Martha, Veracruz did not record the presence of this species (Gonzalez Christen 2008).

The habitat reported by Merriam (1895) for the holotype and paratypes was evergreen tropical forest, represented by well-conserved vegetation that consisted of areas covered by layers of volcanic sand and ashes and trees of large size (Choate 1970, Goldman1951). The lower section of its range, from 1,460 to 1,525 m asl, is within dense primary forest; above this elevation it is montane grassland. The vegetation where the recently discovered specimens were obtained was cloud forest. In one of the three localities sampled, at 1,500m, the trees of the canopy were unusually low (Cervantes and Guevara 2009). It is an insectivorous species, feeding exclusively on insects.

The habitat of C. nelsoni is gradually changing or disappearing due to the use of land within the Los Tuxtlas Biosphere Reserve. The activities within the reserve frequently involve logging, cattle grazing, induced fires, and crops (Cervantes and Guevara 2009) . In the area of the type locality, deforestation is as much as 90% and the annual deforestation rate is 6.2% (Dirzo and Garcia 1992).

Part of the distribution is included in the Los Tuxtlas Biosphere Reserve and Los Tuxtlas Biological Station. According to Cervantes and Guevara (2009), a specific conservation action plan is needed in order to buffer and eventually halt alterations of the habitat and protect the integrity of the populations of this species.